The invention relates to an apparatus for stirring substances and for mixing a compound or a mixture of substances comprising at least two components. In particular, the invention relates to an apparatus for mixing bone cement and filling it into an applicator.
The term "mixing" is generally understood to mean the introduction of the parts of one substance into the parts of other substances. The aim is to obtain as homogenous as possible a distribution of the various components of the mixture, for instance to initiate or promote a chemical reaction between the components such as the following polymerisation of a two-component or multi-component mixture.
Various kinds of mixing processes and mixing devices are known. The process of mixing can be carried out by stirring, mingling, rolling, kneading, emulsifying, suspending, dissolving or by means of ultrasonic waves.
The known mixing processes and mixing apparatus, however, have the disadvantage that impurities, such as air, can enter into the mixing system and that the gas enclosures or occlusions already contained at the beginning of the mixing process in the substances to be mixed and those which form during the actual process of mixing, for instance via a chemical reaction, cannot be removed from the mixing system. A further disadvantage which often occurs is that the mixing vessel is not identical to the vessel in which the mixed compound is to be further processed. It then becomes necessary to transfer the compound. The transferral of the mixed compound might cause problems, especially if reactions such as polymerization occur in the compound.
Further, specific problems arise in the processing and mixing of bone cement and when filling the same into the vessel from which the bone cement is later applied.
The bone cement is usually made of cold-curing two-component resins which anchor the components of artificial joints into the bony bed. The bone cement hardens as soon as it has been applied. Due to its plastic properties, it anchors the components of the prosthesis into the bony bed by interlocking. Polymethylmethacrylates (PMMA) have been used as bone cement for several years now. They comprise a powdery bead polymer which is superficially dissolved in a liquid monomer and is then embedded by the polymerisation of said liquid monomer. In the mixing phase, the monomer surrounds the usually globular polymer powder. This firstly leads to a suspension of the globules in which a considerable amount of air bubbles are entrapped. The process of polymerisation occurs exothermically. In addition to the entrapped air bubbles, when the polymer globules are surrounded by the monomer, so-called "lee phenomena" regularly appear; this means sites where the polymer globules have not been sufficiently wetted. In addition, the monomeric liquid evaporates during the process of exothermal polymerisation which leads to the fact that in the end the hardened bone cement is riddled with bubbles of various ethiology and genesis.
As a rule, the polymer powder is added to the monomer and then mixed in a bowl using a spatula. In the processing phase which follows the mixing phase, the bone cement is applied to the bony bed either manually, which is usually the case, or sometimes with a syringe. To date there are hardly any publications that deal with the mixing phase and bloating phase of the bone cement and with the artefact-free insertion thereof into the syringe system.
The further processing of the bone cement stirred in the mixing bowl in the above-mentioned fashion depends on its viscosity. Bone cement of a very low viscosity can be poured from the bowl into the cartridge of the bone cement syringe; the problem, however, is that the stream of bone cement flowing into the syringe can be very easily diverted, for instance via electrostatic charging, so that the walls of the cartridge and the opening thereof are almost always covered with bone cement. Highly viscous bone cement cannot be poured at all. It has to be removed and kneaded by hand in order to press out the largest of the air enclosures. The bone cement is then rolled into a sausage-shaped mass which can be inserted into the cartridge. When manually processing the cement, not only does one have to wait until the bone cement no longer sticks to the surgical gloves, but the cement mixture also remains unprocessed in the most important stage of the bloating or swelling phase and the pre-polymerisation phase that follows.
The known attempts to try and solve the problem of mixing the cement in a so-called "closed system" have not managed to produce a mixture better than that produced by hand.